Automobiles and railcars use a permanent-magnet rotary electrical machine for a motor or a generator. As the permanent magnet, an Nd—Fe—B magnet is employed to increase efficiency. The magnetic flux density of the Nd—Fe—B magnet is high. Thus, the torque can be high by using an Nd—Fe—B sintered magnet in a rotary electrical machine.
In the motors used for automobiles and railcars, variable speed drive is performed from slow to fast rotation. In conventional motors using Nd—Fe—B sintered magnets, high torque is obtained on the slow rotation side. However, an induced voltage (back electromotive force) is produced on the fast rotation side, and thus, the output is reduced.
In permanent magnets such as Nd—Fe—B sintered magnets, flux linkage is produced with constant strength at any time. The induced voltage by the permanent magnet is increased in proportion to the rate of rotation. Thus, the voltage of the motor reaches the upper limit of the source voltage in fast rotation. The current necessary for output is not supplied. As a result, the output is dramatically reduced. Further, the motor cannot be driven in the range of fast rotation.
To diminish the effect of the induced voltage in fast rotation, for example, a field-weakening control method can be considered. In this method, the magnetic flux density is reduced by producing a reverse magnetic field. In this manner, the number of flux linkages is reduced. However, in permanent magnets having a high magnetic flux density, such as Nd—Fe—B sintered magnets, the magnetic flux density cannot be sufficiently decreased in fast rotation.
Even if a field-weakening control method is used in fast rotation, the effect is insufficient in rotary electrical machines using permanent magnets having a high magnetic flux density.
Embodiments described herein aim to prevent reduction in output in a permanent-magnet rotary electrical machine or vehicle performing variable speed drive from slow to fast rotation.
According to one embodiment, a permanent-magnet rotary electrical machine system comprises a rotary electrical machine, an inverter and a control module. The rotary electrical machine is a permanent-magnet rotary electrical machine which forms a magnetic pole of a rotor with a permanent magnet. The inverter produces an AC voltage by switching and outputs the AC voltage to the rotary electrical machine as drive power. The control module detects a field-component current in the rotary electrical machine, estimates a rate of rotation of the rotor of the rotary electrical machine based on the detected current, obtains a field-component voltage in the rotary electrical machine based on a difference between the estimated rate of rotation of the rotor and a target rate of rotation, and controls the switching of the inverter based on the field-component voltage such that the rate of rotation of the rotor follows the target rate of rotation. The permanent magnet is an R—Co permanent magnet containing 25 to 40 at % iron, where R is at least one element selected from rare-earth elements. The control module performs field-weakening control by increasing and decreasing the field-component voltage based on a negative-field-component current in accordance with the rate of rotation of the rotor by a material of the permanent magnet.